Electrolyte chemistry control in electrodialysis processing

Abstract

Methods for controlling electrolyte chemistry in electrodialysis units having an anode and a cathode each in an electrolyte of a selected concentration and a membrane stack disposed therebetween. The membrane stack includes pairs of cationic selective and anionic membranes to segregate increasingly dilute salts streams from concentrated salts stream. Electrolyte chemistry control is via use of at least one of following techniques: a single calcium exclusionary cationic selective membrane at a cathode cell boundary, an exclusionary membrane configured as a hydraulically isolated scavenger cell, a multivalent scavenger co-electrolyte and combinations thereof.

What is claimed is: 
     
       1. A method for controlling electrolyte chemistry in operation of an electrodialysis unit comprising an anode and a cathode, the anode and the cathode both having an electrolyte solution of a selected concentration continuously applied to both the anode and the cathode and with a membrane stack comprising a plurality of cells in series relative to ion flow disposed between the anode and the cathode, the membrane stack comprising pairs of cationic selective and anionic selective membranes to segregate increasingly dilute salts streams from concentrated salts stream by acting upon a salt solution that is distinct and separate from the electrolyte solution, the method comprising including only one multivalent cation selective exclusionary membrane within the electrodialysis unit and disposing the one multivalent cation selective exclusionary membrane at a cathode cell boundary to limit transport of cationic species to the cathode cell. 
     
     
       2. The method of  claim 1  wherein a) the electrolyte solution comprises disodium sulfate and b) the salt solution comprises cations comprising at least one of calcium, barium, strontium, thorium and radium. 
     
     
       3. The method of  claim 1  wherein the electrolyte solution comprises an elevated pH in a range of 11-12.5 without occurrence of hydroxide salt precipitation. 
     
     
       4. The method of  claim 1  wherein the one multivalent cation selective exclusionary membrane is disposed solely at the cathode cell boundary. 
     
     
       5. The method of  claim 1  wherein the membrane stack comprises at least three pairs of cationic selective and anionic selective membranes. 
     
     
       6. The method of  claim 5  wherein the membrane stack comprises at least 10 pairs of cationic selective and anionic selective membranes. 
     
     
       7. The method of  claim 1  wherein each cell comprises a pairing of a cationic selective membrane and an anionic selective membrane and wherein the flow of anions and cations are caused to become concentrated in one cell pair and diluted in an adjacent cell pair, the method further comprising:
 isolating, from alternate cell pairings, a flow of salt concentrate from a flow of diluate via a concentrate manifold and a diluate manifold, respectively. 
 
     
     
       8. In an electrodialysis unit comprising an anode and a cathode, the anode and the cathode both having an electrolyte solution of a selected concentration continuously applied to both the anode and the cathode and with a membrane stack comprising a plurality of cells in series relative to ion flow disposed between the anode and the cathode, the membrane stack comprising at least three pairs of cationic selective and anionic selective membranes to segregate increasingly dilute salts streams from concentrated salts stream by acting upon a salt solution that is distinct and separate from the electrolyte solution, the improvement comprising:
 the electrodialysis unit containing only one multivalent cation selective exclusionary membrane, with said multivalent cation selective exclusionary membrane disposed solely at a cathode cell boundary to minimize transport of multivalent cations into the cathode cell. 
 
     
     
       9. The improvement of  claim 8  wherein said multivalent cation selective exclusionary membrane minimizes transport of multivalent cations comprising calcium, barium or magnesium. 
     
     
       10. The improvement of  claim 8  wherein a) the electrolyte solution comprises disodium sulfate and b) the salt solution comprises cations comprising at least one of calcium, barium, radium and combinations thereof in high brine conditions. 
     
     
       11. The improvement of  claim 8  wherein the electrolyte solution comprises an elevated pH in a range of 11-12.5 without occurrence of hydroxide salt precipitation. 
     
     
       12. The improvement of  claim 8  wherein the membrane stack comprises at least 10 pairs of cationic selective and anionic selective membranes. 
     
     
       13. The improvement of  claim 8  wherein each cell comprises a pairing of a cationic selective membrane and an anionic selective membrane and wherein the flow of anions and cations are caused to become concentrated in one cell pair and diluted in an adjacent cell pair, the method further comprising:
 isolating, from alternate cell pairings, a flow of salt concentrate from a flow of diluate via a concentrate manifold and a diluate manifold, respectively. 
 
     
     
       14. A method for controlling electrolyte chemistry in an electrodialysis unit, the electrodialysis unit comprising an anode and a cathode, with an electrolyte solution of a selected concentration continuously applied to both the anode and the cathode, and wherein a membrane stack comprising a plurality of cells in series relative to ion flow is disposed between the anode and the cathode to segregate increasingly dilute salts streams from concentrated salts stream by acting upon a salt solution that is distinct and separate from the electrolyte solution, the method comprising:
 the electrodialysis unit containing only one multivalent cation selective exclusionary membrane, said multivalent cation selective exclusionary membrane disposed at a cathode cell boundary and 
 wherein each cell comprises a pairing of a cationic selective membrane and an anionic selective membrane and wherein the flow of anions and cations are caused to become concentrated in one cell pair and diluted in an adjacent cell pair, the method further comprising: 
 isolating, from alternate cell pairings, a flow of salt concentrate from a flow of diluate via a concentrate manifold and a diluate manifold, respectively.